Acoustic Tractor Beam

A system that uses sound waves to manipulate objects in mid air, and even inside the body, has been developed by scientists in the UK.

Fans of Star Trek and similar sci-fi genres will be acquainted with the concept of the tractor beam, and "we've invented the world's first working one," says Bristol University's Bruce Drinkwater.

The approach they've taken, which is published this week in the journal Nature Communications, comprises a hand-sized oblong array of 64 ultrasonic loudspeakers.

These emit sound waves at about 40 kHz, which is similar in pitch to the high-frequency squeaks of a bat but totally inaudible for a human.

"This means we can actually make the sounds incredibly loud and powerful, but the experiments are essentially silent," explains Drinkwater. "Although cats and dogs probably won't like it!"

The sound waves issuing from the array interact, or interfere, with each other in the air in front of the speakers.

In some places the waves sum together to produce the equivalent of a stormy sea, while in other adjacent regions the waves cancel out to produce a calm spot.

Objects placed within the influence of the waves naturally tend to seek out the flat spots and remain there.

"It's the 3d sound equivalent of a hologram," says Drinkwater. "We've actually got several different forms of sonic cage for holding things like this, including tweezers, a vortex or tornado-like effect, and a bottle-shaped receptacle."

By delicately changing the patterns of the sound waves that make up the 3d soundscape, the cage can also be moved about, enabling the objects trapped inside to be manipulated upwards, downwards and side to side, hence the reference to a tractor beam.

The wavelengths of the ultrasound waves used in the experiment mean that the team can currently manipulate objects that are up to the size of a small pea, although they are working on a version that would enable them to move something football-sized in future.

"But what I am really interested in doing is applying this inside the human body," says Drinkwater. "Sound travels very well through tissue, so we could use this to control devices internally."

So surgery could be conducted internally in future, using tiny sound-guided scalpels.

"In the nearer term though, we're looking at a way to guide packets of drugs to the precise spots we wanted them and then a burst of ultrasound to release them only where they were needed." Sounds like a good idea...

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